fix(core): follow merged tombstones in the position lookup

A merged child still holds its (parent_id, position) slot under the unique
index, so skipping tombstones made every retry collide. Follow the tombstone
to the surviving work instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/test/matching.test.ts

@@ -3,7 +3,7 @@ import type { Db } from "@sttracker/db/db";
 import { newId, nowIso } from "@sttracker/db/ids";
 import { forEachDialect } from "@sttracker/db/testing";
 import { fuzzyScore, matchWork } from "../src/matching.ts";
-import { createWork, getWork } from "../src/works.ts";
+import { adminMergeWorks, createWork, getWork } from "../src/works.ts";
 
 async function addUser(db: Db): Promise<string> {
   const id = newId();
@@ -145,4 +145,19 @@ forEachDialect("matching", (getDb) => {
     await db.k.insertInto("work_aliases").values({ id: newId(), user_id: u, from_work_id: shared, to_work_id: priv }).execute();
     expect(await matchWork(db, u, { title: "Dune", mediaType: "book", externalIds: { isbn13: "1" } }, "p")).toEqual({ workId: priv, created: false, draft: true });
   });
+
+  test("position lookup follows a merged tombstone instead of colliding with it", async () => {
+    const db = getDb();
+    const u = await addUser(db);
+    await db.k.updateTable("users").set({ role: "admin" }).where("id", "=", u).execute();
+    const e1 = await matchWork(db, u, chain(), "p");
+    const h2 = chain();
+    h2.title = "Episode 2";
+    h2.position = { season: 1, episode: 2 };
+    const e2 = await matchWork(db, u, h2, "p");
+    await adminMergeWorks(db, u, e1.workId, e2.workId);
+    const before = (await db.k.selectFrom("works").select("id").execute()).length;
+    expect(await matchWork(db, u, chain(), "p")).toEqual({ workId: e2.workId, created: false, draft: false });
+    expect(await db.k.selectFrom("works").select("id").execute()).toHaveLength(before);
+  });
 });